What does "keeps an oath even when it hurts" (15:4) mean?

In Hebrew keeps an oath is the verb shava, which Brown-Driver-Briggs gives in this form as "swear, take an oath" (tap oath). Even when it hurts is a form of the verb Brown-Driver-Briggs gives, in that stem, as "do an injury, hurt". And doesn’t change is mur, "change" (Brown-Driver-Briggs), the verb the Law uses for a man who might exchange an animal given to the LORD: "He shall not alter it, nor exchange it" (Leviticus 27:10), where exchange is mur (tap change). The psalm doesn't say which oaths it means. The Law says of a man who "swears an oath to bind his soul with a bond, he shall not break his word" (Numbers 30:2). Ecclesiastes says of vows, "It is better that you should not vow, than that you should vow and not pay" (Ecclesiastes 5:5). And when the Gibeonites were found out, "all the princes" (Joshua 9:19) said: "We have sworn to them by the LORD, the God of Israel. Now therefore we may not touch them" (Joshua 9:19), where sworn is shava. The psalm doesn't name that oath, and Joshua 9's own honest question weighs it.